[chore:] main-window accessible from outside

MainWindow.axaml.cs

@@ -10,10 +10,12 @@ namespace spplus;
 
 public partial class MainWindow : Window
 {
+    public static MainWindow Instance { get; set; }
     public MainWindow()
     {
         InitializeComponent();
         Settings.ImportInitial();
+        Instance = this;
     }
 
     private void MnuExpSettings_OnClick(object? sender, RoutedEventArgs e)
@@ -115,7 +117,6 @@ public partial class MainWindow : Window
         LblStudentAmount.Content =  Settings.Instance.Students.Count.ToString();
         LblSelectedAmount.Content =  count_selected.ToString();
         
-        
     }
 
     private void BtnCraftCourses_OnClick(object? sender, RoutedEventArgs e)